Hydrophilicizing post-treatment over chromate conversion coating

ABSTRACT

Substantially improved hydrophilicity can be provided to a metal substrate, particularly an aluminum alloy of the type used for heat exchanger fins, that has been previously provided with a conventional chromium oxide conversion coating, by drying into place on the surface of the chromium oxide conversion coating an aqueous liquid hydrophilicizing treatment composition containing at least one of: (i) aproduct or products of reaction between dissolved phosphoric acid and at least one of elemental metal, metal oxides, and metal hydroxides in contact therewith; (ii) tungstate ions; and (iii) at least one of sulfate or molybdate ions together with polyethyleneimine. Most preferably, a solution formed by dissolving MgO in aqueous H 3  PO 4  in the proper stoichiometric amount to form an aqueous solution of Mg(H 2  PO 4 ) 2  with a pH of 4.5-5.5 is used for the hydrophilicizing treatment.

CROSS-REFERENCE TO RELATED APPLICATION

Priority for this application is claimed under 35 U.S.C. § 119(e) fromapplication Ser. No. 60/008761 filed Dec. 13, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a hydrophilicizing treatment for a chromateconversion coating over underlying metal, particularly aluminum or analuminum alloy containing at least 75% of aluminum. After treatment of asurface according to this invention, water will have a tendency tospread spontaneously over the surface. The invention is particularlyadvantageously applicable to provide aluminum evaporators, heatexchangers, and condensers, especially those used for automobile airconditioning, with hydrophilic coatings that have good corrosionresistance and a durably hydrophilic character with little or notendency to develop undesirable odors during use.

2. Statement of Related Art

Although any of the common structural metals can be used in constructingpractical heat exchanging surfaces, aluminum and its alloys are amongthose most often used, because of their high heat conductivity. In heatexchanger surfaces, metals are normally used without any relativelythick protective coating such as a paint or lacquer that would normallybe used to improve the resistance of the metals to corrosion in mostother types of metal structures, but is avoided in heat exchangersbecause such a coating would also reduce the efficiency of heatexchange.

During the cooling of hot air, a common use of these heat exchangers,moisture contained as vapor in the hot air condenses and initially formswater drops or beads on the fins of the heat exchanger. If the surfaceof the heat exchanger fins is not sufficiently hydrophilic, the waterbeads accumulate on the fin surface and impede the air flow between finsand also reduce the heat transfer efficiency. The condensed water beadsalso tend to absorb dust and contaminants in the air, such as carbondioxide, nitrogen oxides, and sulfur oxides, which can promote corrosionof the underlying aluminum. Therefore, the formation of water beads onthe fins of aluminum heat exchanger not only decreases heat transferefficiency but also can physically damage the exchanger.

In order to achieve a desirable combination of a hydrophilic nature andcorrosion resistance on metal, particularly aluminum, surfaces, variouscoatings and treatments have been tried, but no fully satisfactoryresult has yet been achieved. A chromate conversion coating without anypost-treatment usually has inadequate corrosion resistance and oftendevelops an unpleasant odor. Silicate coating over a chromate conversioncoat has often been used but has not satisfied all users. More recently,biocide protected hydrophilic organic polymer films have been used aspost-treatments over chromate conversion coatings. While effective,these have proved to be expensive and difficult to control in somecommercial operations.

DESCRIPTION OF THE INVENTION

Object of the Invention

The principal object of the invention is to provide metal heat exchangersurfaces with adequate hydrophilicity and corrosion resistance, whileavoiding or at least mitigating at least one of the following problemssometimes observed with prior art hydrophilicizing treatments:dissolution of environmentally undesirable hexavalent chromium from theunderlying chromate coating; dust in the passenger compartment ofautomobiles equipped with air conditioners bearing a prior art coatingon its heat exchangers; unpleasant odors in the vicinity of the heatexchanger surface when operating; a need for expensive materials to formthe hydrophilic surface; and complicated maintenance requirements forthe process of forming the hydrophilic coating. Other objects will beapparent from the description below.

General Principles of Description

Except in the claims and the operating examples, or where otherwiseexpressly indicated, all numerical quantities in this descriptionindicating amounts of material or conditions of reaction and/or use areto be understood as modified by the word "about" in describing thebroadest scope of the invention. Practice within the numerical limitsstated is generally preferred, however. Also, unless expressly stated tothe contrary: percent, "parts of", and ratio values are by weight; thedescription of a group or class of materials as suitable or preferredfor a given purpose in connection with the invention implies thatmixtures of any two or more of the members of the group or class areequally suitable or preferred; description of constituents in chemicalterms refers to the constituents at the time of addition to anycombination specified in the description, and does not necessarilypreclude chemical interactions among the constituents of a mixture oncemixed; specification of materials in ionic form implies the presence ofsufficient counter-ions to produce electrical neutrality for thecomposition as a whole; any counterions thus implicitly specified shouldpreferably be selected from among other constituents explicitlyspecified in ionic form, to the extent possible; otherwise suchcounterions may be freely selected, except for avoiding counterions thatact adversely to the objects of the invention; the terms "molecule" and"mole" and their grammatical variations may be applied to ionic,elemental, or any other type of chemical entities defined by the numberof atoms of each type present therein, as well as to substances withwell-defined neutral molecules; the first definition of an acronym orother abbreviation applies to all subsequent uses herein of the sameabbreviation; and the term "polymer" includes "oligomer", "homopolymer","copolymer", "terpolymer", and the like.

SUMMARY OF THE INVENTION

It has been found that a desirable combination of hydrophilicity andcorrosion resistance can be achieved on the surface of metals,particularly aluminum and its alloys containing at least 75% by weightaluminum, by first providing the metal surface with a conventionalchromium oxide conversion coating, and then, preferably very soon afterforming the conversion coating, post-treating the surface with at leastone of (i) a solution having a composition which could be achieved, andwhich preferably actually is made, by dissolving elemental metal, metaloxides, and/or metal hydroxides in aqueous phosphoric acid, (ii) anaqueous solution containing tungstate ions, and (iii) an aqueoussolution containing sulfate or molybdate ions together withpolyethyleneimine.

One embodiment of the invention is a composition suitable for direct usein imparting hydrophilicity to a chromated metal surface; such acomposition may be described hereinafter as a "working composition".Another embodiment of the invention is a concentrate composition, whichcan be diluted with water to produce a working composition as specifiedabove. Still another embodiment of the invention is a process oftreating a chromated metal surface as described above.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The preferred aqueous treatment compositions of the invention, whichcontain phosphoric acid at least as a starting material, may utilize anyof the phosphoric acids in which phosphorus is in its +5 oxidationstate, i.e., metaphosphoric acid (HPO₃), orthophosphoric acid (H₃ PO₄),pyrophosphoric acid (H₄ P₂ O₇), or any of the higher condensedphosphoric acids with the general formula H.sub.(n+2) P_(n)O.sub.(3n+1), where n represents a positive integer with a value of atleast three. It is generally believed that all these acids are inequilibrium with one another in aqueous solutions, with orthophosphoricacid being much the most predominant at low concentrations andtemperatures and the more condensed acids (including metaphosphoricacid) becoming important only at high concentrations and temperatures,or when their salts are present in the aqueous solutions along withacid. At least partly for reasons of economy, orthophosphoric acid isgenerally preferred for use in this invention.

Aqueous solutions of any salts of phosphoric acid(s) can be used aspost-treatments according to the invention, but, at least for thepreferred salts, it is preferable to utilize aqueous treatmentcompositions prepared by dissolving metal oxides and/or hydroxides inaqueous phosphoric acid solutions rather than solutions of the saltsthemselves. The preferred treatment solutions are described in this waybecause it is often possible to obtain transparent and otherwiseapparently stable solutions by dissolving metal oxides and/or hydroxidesin aqueous phosphoric acid, even though these apparent solutions are"supersaturated" with respect to the phosphate and/or mono- or di-acidphosphate salt or salts to which their phosphoric acid and metalcontents nominally correspond. Although the invention is not to beconsidered to be limited by any theory, it is believed that these"supersaturated" solutions may contain coordination compounds or otherchemical species of unknown structure that are at least part of thereason for their hydrophilicizing properties.

Preferably the metal oxides codissolved with phosphoric acid in theaqueous liquid post-treatment compositions used according to theinvention are oxides of metals with a valence of at least two, morepreferably exactly two. The single most preferred metal is magnesium.

When the oxide or hydroxide of one or more divalent metals is used withorthophosphoric acid as preferred, the molar ratio of the divalent metalto the phosphorus atoms in the orthophosphoric acid preferably is atleast, with increasing preference in the order given, 1.0:5.0, 1.0:4.0,1.0:3.5, 1.0:3.0, 1.0:2.8, 1.0:2.6, 1.0:2.4, 1.0:2.3, 1.0:2.2, 1.0:2.1,or 1.0:2.05 and independently preferably is not more than 1.0:0.5,1.0:0.8, 1.0:1.0, 1.0:1.2, 1.0:1.4, 1.0:1.6, 1.0:1.7, 1.0:1.8, 1.0:1.9,or 1.0:1.95.

Independently, at the time of mixing of a metal oxide and/or hydroxidewith aqueous phosphoric acid to form an aqueous liquid post-treatingcomposition according to the invention, the concentrations of thephosphoric acid and divalent metal oxides and/or hydroxides arepreferably such that the concentration of the corresponding metalmonohydrogen, dihydrogen, or non-hydrogen-containing phosphate salt orsalts is at least, with increasing preference in the order given, 1, 3,5.0, 6.0, 7.0, 8.0, 8.5, 9.0, 9.3, 9.5, 9.7, or 9.9% and independentlypreferably is not more than, with increasing preference in the ordergiven, 80, 65, 50, 40, 35, 30, 25, 20, 17, 15, 13, 12.0, 11.5, 11.0,10.7, 10.5, 10.3, or 10.1%. A solution prepared in this way serves as anexcellent concentrate composition according to the invention. In aworking composition according to the invention, however, theconcentration of dissolved metal monohydrogen, dihydrogen, ornon-hydrogen-containing phosphate salt or salts preferably is not morethan, with increasing preference in the order given, 50, 40, 30, 20, 17,15, 13, 12, 11, or 10.5% of the preferred concentration of the sameconstituents at the time of mixing of aqueous phosphoric acid and metaloxide and/or hydroxide. Independently, primarily for reasons of economy,in a working composition according to the invention the concentration ofdissolved metal monohydrogen, dihydrogen, or non-hydrogen-containingphosphate salt or salts preferably is at least, with increasingpreference in the order given, 1, 3, 5, 6.0, 7.0, 7.5, 8.0, 8.5, 9.0,9.3, 9.5. 9.7, or 9.9% of the preferred concentration of the sameconstituents at the time of mixing of aqueous phosphoric acid and metaloxide and/or hydroxide to form a concentrate composition according tothe invention.

The pH of a working hydrophilicizing composition according to thisinvention preferably is at least, with increasing preference in theorder given, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.2, or 5.4 andindependently preferably is not more than, with increasing preference inthe order given, 8.0, 7.5, 7.0, 6.5, 6.0, 5.8, 5.6, or 5.5. The pH maybe adjusted by slightly varying the amounts of metal oxide or hydroxidemixed with phosphoric acid to make a preferred type of compositionaccording to the invention, or by other means known in the art.

The technically satisfactory embodiments of the invention that do notinclude any use of phosphate ions are substantially more expensive thanthe most preferred compositions, and are generally less preferred forthat reason. In addition some of these less preferred alternatives givegood hydrophilicity only under limited conditions, as discussed below inconnection with the examples. If a polyalkyleneimine polymer is used, itpreferably is a polyethyleneimine polymer.

A process according to the invention preferably produces a surface onwhich the advancing contact angle of deionized or other at least equallypurified water in air is not more than, with increasing preference inthe order given, 45, 40, 35, 32, or 29 degrees.

For best results, a hydrophilicizing process according to the inventionshould be applied promptly after the chromate conversion coating to behydrophilicized has been formed. More specifically, the time intervalbetween the end of formation of the chromate conversion coating,including any time of rinsing the initially formed coating with water,and the beginning of hydrophilicizing treatment according to theinvention preferably is not more than, with increasing preference in theorder given, 600, 300, 200, 150, 100, 90, 80, 70, 60, 50, 40, 35, 30,25, 20, or 15 seconds. Independently, it is preferred that no deliberatedrying of the chromate conversion coating, other than that which occursinherently during any interval of dwell in ambient air between theformation of this conversion coating and the beginning ofhydrophilicizing treatment as described above, and no other heating ofthe chromate conversion coated surface, be allowed to occur between theformation of the chromate conversion coating and beginning ahydrophilicizing treatment according to this invention.

Independently of other preferences, the increase in mass per unit areatreated in a hydrophilicizing process according to the inventionpreferably is at least, with increasing preference in the order given,0.1, 0.30, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, or 0.74 grams persquare meter (hereinafter usually abbreviated as "g/m² ") andindependently preferably is, primarily for reasons of economy, not morethan, with increasing preference in the order given, 10, 5, 4.0, 3.0,2.5, 2.0, 1.7, 1.4, 1.2, 1.0, or 0.8 g/m².

For various reasons it is often preferred that compositions according tothe invention as defined above should be substantially free from manyingredients used in compositions for similar purposes in the prior art.Specifically, it may be increasingly preferred in the order given,independently for each preferably minimized component listed below, thatthese compositions, when directly contacted with metal in a processaccording to this invention, contain no more than 1.0, 0.35, 0.10, 0.08,0.04, 0.02, 0.01, or 0.001% of each of the following constituents:ferricyanide, ferrocyanide, nitrate and other oxidizing agents (theothers being measured as their oxidizing stoichiometric equivalent asnitrate), organic liquids with a boiling point below 120° C., organiccompounds containing amino and/or amido groups, undissolved dispersedsilica, and dissolved or dispersed silicates.

The aluminum or aluminum alloy substrates to be treated according tothis invention should first be provided with a conventional chromiumcontaining conversion coating treatment of the type generally known inthe art as "chromium oxide" treatments. These are illustrated, withoutlimitation, by U.S. Pat. No. 4,146,410. While any treatment thateffectively provides a chromium oxide conversion coating may be used toform the base substrate that is hydrophilicized by a process accordingto this invention, preferred chromium oxide conversion coating formingliquid compositions comprise, preferably consist essentially of, or morepreferably consist of, water and:

(A) CrO₃ in a concentration that is at least, with increasing preferencein the order given, 0.6, 0.8, 1.0, 1.3, 1.5, 1.7, 1.8, or 1.85 grams perliter (hereinafter usually abbreviated as "g/L") and independentlypreferably is not more than, with increasing preference in the ordergiven, 10, 7.0, 5.0, 4.0, 3.5, 3.0, 2.6, 2.2, 2.0, or 1.90 g/L; and

(B) sufficient strong acid to provide at least, with increasingpreference in the order given, 0.5, 1.0, 1.4, 1.8, 2.1, 2.4, 2.7, 3.0,3.3, or 3.5 points of free acid¹ and independently, primarily forreasons of economy, preferably not more than, with increasing preferencein the order given, 100, 50, 25, 15, 10, 7.0, 5.0, 4.5, 4.2, 3.9, or 3.7points of free acid; and, optionally and preferably, one or both, morepreferably both, of the following constituents:

(C) simple or complex fluoride ions in a total concentration, measuredas its stoichiometric equivalent as HF, of at least, with increasingpreference in the order given, 0.10, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45,0.48, 0.51, or 0.53 g/L and independently preferably is not more than,with increasing preference in the order given, 7, 5, 4, 3.0, 2.5, 2.0,1.5, 1.0, 0.90, 0.80, 0.70, 0.65, 0.61, 0.58, or 0.55 g/L; and

(D) a component selected from the group of tungstate and molybdate ionsin a concentration, measured as its stoichiometric equivalent as Na₂WO₄.2H₂ O with any molybdenum present being counted as the same numberof tungsten atoms to determine the stoichiometric equivalent, that is atleast, with increasing preference in the order given, 0.05, 0.10, 0.15,0.20, 0.25, 0.30, 0.35, 0.38, or 0.41 g/L and independently, primarilyfor reasons of economy, preferably is not more than, with increasingpreference in the order given, 7, 5, 3.0, 2.0, 1.4, 0.90, 0.80, 0.70,0.60, 0.50, 0.46, or 0.43 g/L; and, optionally but not necessarilypreferably, one or more of the following constituents:

(E) chromium(III) cations;

(F) a reducing agent in an amount sufficient to reduce some but not allof the chromium added as component (A) to provide chromium(III) cations;and

(G) a component of compounds of one or more of boron, silicon, titanium,and zirconium that react with hydrofluoric acid to form complex fluorideanions.

It is normally preferred that component (A) be supplied by addition ofCrO₃ itself, but the stoichiometric equivalent amount of hexavalentchromium can also be supplied by chromate and dichromate salts. Up to60% of the chromium atom content can be reduced to chromium(III) ifdesired, by addition of component (F). If such a mixed Cr(III) andCr(VI) conversion coating solution is used, the stoichiometricequivalent as CrO₃ of all chromium atoms present in the compositionshould be considered as forming part of the numerical values specifiedas preferred for component (A). Preferably, primarily for reasons ofeconomy, with increasing preference in the order given, not more than50, 40, 30, 20, 10, or 5% of the chromium atoms present in thecomposition are trivalent, with the remainder hexavalent.

Any acid with a larger ionization constant than that of hydrofluoricacid can be used for component (B), but nitric acid is generallypreferred, inasmuch as sulfuric, phosphoric, and hydrochloric acids, theother relatively inexpensive strong acids, are at least mildlydeleterious to the desired effectiveness of the chromate conversionscoating compositions.

The presence of component (C) is generally highly preferred, andpreferably at least, with increasing preference in the order given, 10,20, 30, 40, 50, 60, 70, 80, or 90% of the total fluorine atoms presentin component (C) are present as complex rather than simple fluorideions. The complex fluoride ions may be obtained directly from additionsof fluoboric, fluosilicic, fluotitanic, or fluozirconic acids and theirsalts, or, often more economically, by adding hydrofluoric acid and someother compound of one of the elements specified for optional component(G), such as boric acid, silica, or the like.

The presence of optional component (D) is generally highly preferred,inasmuch as the conversion coating forming process is usuallyundesirably slow when no material that satisfies the requirements forcomponent (D) is present.

The use of component (E) when desired is well known in the art. Lowmolecular weight (and thus readily water-soluble) alcohols, saccharides,formaldehyde, and/or the like are generally used. No substantialadvantage or disadvantage from using part of the chromium content of acomposition according to the invention in trivalent rather thanhexavalent form, within the preferred limits already given above, hasbeen observed.

The mass per unit area of surface treated added-on by the conversioncoating treatment preferably is at least, with increasing preference inthe order given, 0.10, 0.30, 0.50, 0.70, 0.80, 0.90, 0.95, 1.00, or 1.05g/m² and independently preferably is not more than, with increasingpreference in the order given, 10, 7, 5.0, 4.0, 3.0, 2.0, 1.7, 1.5,1.30, 1.20, 1.15, or 1.10 g/m² of surface treated. Before the conversioncoating treatment, the surface(s) to be treated are preferably cleanedof any foreign matter and then rinsed, in one of the many ways known inthe art to be effective.

The practice and benefits of the invention may be further appreciated byconsideration of the following non-limiting working and comparisonexamples.

GENERAL CONDITIONS FOR THE EXAMPLES AND COMPARISON EXAMPLES

The surfaces treated were those on sample sheets (panels) of Type 3003aluminum alloy. The processing sequence for all experiments included thefollowing steps:

1. Acid cleaning by immersion of the sample sheets in a solution of 1.5%by volume of a concentrated acid cleaner formulation in water at 38° C.for 2.0 minutes.

2. Rinse twice, for 20 seconds each time, with cold tap water.

3. Chromate conversion coating by immersion in a 4% by weight solutionin water of ALODINE® 713 Chromium Conversion Coating concentrate at 38°C. for 180 sec to produce an add-on mass of 1.0-1.1 g/m².

4. Rinse with cold tap water for 30 sec.

5. Hydrophilicizing treatment at 20-25° C. for 60 sec by immersion ofthe samples, producing a wet coating mass of about 0.75 g/m².(Initially, tests both with and without rinsing after this step wereperformed, but those with rinsing generally produced less hydrophilicsurfaces, so that all the results shown below are for substratesprocessed without rinsing between this step and step 6.)

6. Allow to drain for 15 seconds in air. (If the surfaces to be treatedhave complicated shapes, so that they do not drain well, mechanicalshaking, centrifugation, or the like is advantageous during the drainingstep.)

7. Oven dry at 120° C. for 15 minutes.

ALODINE® 713 concentrate is commercially available from the ParkerAmchem Div. ("PAM") of Henkel Corp., Madison Heights, Mich., U.S.A. andis an example of a concentrate for a chromium oxide type conversioncoating treatment. Prior art conventional hydrophilicizing treatmentsare exemplified by PALENE® 210 concentrates, available from NihonParkerizing Company, Ltd., Tokyo, Japan and/or PAM.

Other hydrophilicizing treatment compositions identified below assolutions of phosphate salts were in fact made by dissolving thecorresponding metal oxide or hydroxide in aqueous orthophosphoric acidto neutralize part of the acid and produce a concentration of 10%stoichiometric equivalent as the salt shown in the identification as theonly deliberately added ingredients in a concentrate, which was thendiluted with additional water to form a working composition. Othertreatment compositions identified below as solutions of specificchemicals were made by dissolving the actual chemical shown in theidentifications.

The advancing contact angle of deionized water against the surfaceprepared by the treatments described above was determined by use of aRame-Hart Model 100-00 NRL Contact Goniometer at a time 24 to 48 hoursafter the completion of the hydrophilicizing treatment and cooling ofthe treated substrate surface and also after (i) these surfaces had beenimmersed in flowing deionized water, displaced at a rate of 10% byvolume of the total volume of the overflowable container in which thesamples were immersed per minute, for 100 hours at ambient temperature,then dried for 2 hours in ambient air after removal from the containerof flowing deionized water or (ii) the surfaces had been exposed for 168hours to water vapor saturated air (i.e., 100% relative humidity) at 38°C., then rinsed with water and dried for 2 hours at ambient temperature.

Treating compositions and results with them are shown in Table 1 below.The results in Table 1 indicate that sulfate and molybdate salts areeffective hydrophilicizing treatments over chromium oxide conventioncoatings, but only if they are used together with polyalkyleneiminepolymer. Tungstate and sodium phosphate give good hydrophilicity alone,but only after immersion in water or exposure to humid air. However,tungstate is far more expensive than either type of phosphate, andmagnesium phosphate gives good hydrophilicity immediately as well asafter exposure to water or water vapor. Therefore, magnesium phosphateis most preferred.

                  TABLE 1                                                         ______________________________________                                        Aqueous Solution Tested for                                                                     Contact Angle in Degrees After:                             Hydrophilicizing Effect                                                                         Treatment                                                                              + Immer. + Humid.                                  ______________________________________                                        1.0% Na.sub.2 WO.sub.4.2H.sub.2 O                                                               45       11       25                                          1.0% Na.sub.2 WO.sub.4.2H.sub.2 O + 33 10 46                                  1% POLYMIN ™ P                                                             1.0% Na.sub.2 MoO.sub.4.3H.sub.2 O 55 30 33                                   1.0% Na.sub.2 MoO.sub.4.3H.sub.2 O + 19 10 50                                 1% POLYMIN ™ P                                                             1.0% Mg(H.sub.2 PO.sub.4).sub.2, pH 5.3 10 15 29                              1.0% NaH.sub.2 PO.sub.4, pH 5.3 55  9 25                                      1.0% MgSO.sub.4, pH 5.3 37 39 34                                              1.0% MgSO.sub.4 + 1% POLYMIN ™ P 28 18 30                                  0.5% MoO.sub.3 + NH.sub.4 OH.sup.1 + 22  9 22                                 1% POLYMIN ™ P                                                             No treatment 64 45 56                                                         PALENE ® 210 hydrophilicizing 40 21 30                                    treatment composition                                                       ______________________________________                                         Abbreviations, General Notes, and Footnote for Table 1                        Immer. = Immersion; Humid. = Humidity Exposure; POLYMIN ™ P is a           commercial product of BASF Corp. and is reported by its supplier to           contain 49% of polyethyleneimine with an average molecular weight of abou     70,000, 1% of polyethyleneimine hydrochloride, and the balance water.         .sup.1 The amount of aqueous ammonia added was just sufficient to clarify     a suspension of the previously added MoO.sub.3.                          

The invention claimed is:
 1. A process for increasing the hydrophilicityof a surface of a chromium oxide conversion coating on a metal substratethat contains at least 75% of aluminum, said process comprising stepsof:(I) forming on said surface a liquid layer of a hydrophilicizingaqueous solution comprising water and at least one of:(i) a product orproducts of reaction between dissolved phosphoric acid and at least oneof elemental metal, metal oxides, and metal hydroxides in contacttherewith; (ii) tungstate ions; and (iii) at least one of sulfate ormolybdate ions together with polyethyleneimine, said hydrophilicizingaqueous solution not comprising more than 0.10% of dissolved ordispersed silicates; and (II) drying into place on the surface theliquid layer formed in step (I), so as to form with the chromium oxideconversion coating an adherent solid hydrophilic coating over the metalsubstrate.
 2. A process according to claim 1, wherein saidhydrophilicizing aqueous solution comprises from about 0.3 to about 8.0%of a dissolved product of reaction between (1) a number of moles,measured as its stoichiometric equivalent as phosphorus atoms, ofphosphoric acid and (2) a total number of moles, measured as itsstoichiometric equivalent as metal atoms, of material selected from thegroup consisting of (2.1) hydroxides of metals and (2.2) oxides ofmetals, in amounts of reagents (1) and (2) such that the total number ofmoles of reagent (2) has a ratio to the number of moles of phosphorusatoms of reagent (1) that is in a range from about 1.0:5.0 to about1.0:0.5.
 3. A process according to claim 2, wherein saidhydrophilicizing aqueous solution comprises from about 0.6 to about 4.0%of a product of reaction between (1) a number of moles, measured as itsstoichiometric equivalent as phosphorus atoms, of phosphoric acid and(2) a total number of moles, measured as its stoichiometric equivalentas divalent metal atoms, of material selected from the group consistingof (2.1) hydroxides of divalent metals and (2.2) oxides of divalentmetals, in amounts of reagents (1) and (2) such that the total number ofmoles of reagent (2) has a ratio to the number of moles of phosphorusatoms of reagent (1) that is in a range from about 1.0:3.5 to about1.0:1.0.
 4. A process according to claim 3, wherein saidhydrophilicizing aqueous solution has a pH from about 5.0 to about 5.5and comprises from about 0.8 to about 1.2% of a product of reactionbetween (1) a number of moles, measured as its stoichiometric equivalentas phosphorus atoms, of phosphoric acid and (2) a total number of moles,measured as its stoichiometric equivalent as magnesium atoms, ofmaterial selected from the group consisting of (2.1) magnesium hydroxideand (2.2) magnesium oxide, in amounts of reagents (1) and (2) such thatthe total number of moles of reagent (2) has a ratio to the number ofmoles of phosphorus atoms of reagent (1) that is in a range from about1.0:2.2 to about 1.0:1.8.
 5. A process according to claim 4,wherein:step (I) is begun not more than about 30 seconds after saidchromium oxide conversion coating has been formed on said metalsubstrate and before said chromium oxide conversion coating has everdried since it was formed; said chromium oxide conversion coating has adry mass per unit area from about 0.70 to about 1.30 g/m² ; the liquidlayer formed in step (I) contains from about 0.50 to about 1.7 g/m² ofmaterial that remains in place on the surface after drying; anddeionized water has an advancing contact angle that is not more than 35°on the surface formed after step (II) has been completed.
 6. A processaccording to claim 3, wherein:step (I) is begun before said chromiumoxide conversion coating has ever dried since it was formed; saidchromium oxide conversion coating has a dry mass per unit area fromabout 0.30 to about 5.0 g/m² ; the liquid layer formed in step (I)contains from about 0.30 to about 4.0 g/m² of material that remains inplace on the surface after drying; and deionized water has an advancingcontact angle that is not more than 45° on the surface formed after step(II) has been completed.
 7. A process according to claim 2, wherein:step(I) is begun before said chromium oxide conversion coating has everdried since it was formed; and deionized water has an advancing contactangle that is not more than 45° on the surface formed after step (II)has been completed.
 8. A process according to claim 1, wherein:step (I)is begun before said chromium oxide conversion coating has ever driedsince it was formed; and deionized water has an advancing contact anglethat is not more than 45° on the surface formed after step (II) has beencompleted.
 9. A concentrated aqueous liquid composition of matter,suitable for dilution with water to form an aqueous solution that, whenapplied as a layer over a surface of a chromium oxide conversion coatingand dried into place thereover, will form a surface on which deionizedwater has a smaller advancing contact angle than it has on the surfaceof the chromium oxide conversion coating, said composition consistingessentially of water and from about 5.0 to about 40% of a product ofreaction between (1) a number of moles, measured as its stoichiometricequivalent as phosphorus atoms, of phosphoric acid and (2) a totalnumber of moles, measured as its stoichiometric equivalent as divalentmetal atoms, of material selected from the group consisting of (2.1)hydroxides of divalent metals and (2.2) oxides of divalent metals, inamounts of reagents (1) and (2) such that the total number of moles ofreagent (2) has a ratio to the number of moles of phosphorus atoms ofreagent (1) that is in a range from about 1.0:3.5 to about 1.0:1.0. 10.A concentrated aqueous liquid composition of matter according to claim9, said composition consisting essentially of water and from about 6.0to about 30% of a product of reaction between (1) a number of moles,measured as its stoichiometric equivalent as phosphorus atoms, ofphosphoric acid and (2) a total number of moles, measured as itsstoichiometric equivalent as divalent metal atoms, of material selectedfrom the group consisting of (2.1) hydroxides of divalent metals and(2.2) oxides of divalent metals, in amounts of reagents (1) and (2) suchthat the total number of moles of reagent (2) has a ratio to the numberof moles of phosphorus atoms of reagent (1) that is in a range fromabout 1.0:2.6 to about 1.0:1.6.
 11. A concentrated aqueous liquidcomposition of matter according to claim 10, said composition consistingessentially of water and from about 8.5 to about 11.0% of a product ofreaction between (1) a number of moles, measured as its stoichiometricequivalent as phosphorus atoms, of phosphoric acid and (2) a totalnumber of moles, measured as its stoichiometric equivalent as divalentmetal atoms, of material selected from the group consisting of (2.1)hydroxides of divalent metals and (2.2) oxides of divalent metals, inamounts of reagents (1) and (2) such that the total number of moles ofreagent (2) has a ratio to the number of moles of phosphorus atoms ofreagent (1) that is in a range from about 1.0:2.2 to about 1.0:1.9. 12.A concentrated aqueous liquid composition of matter according to claim11, wherein reagent (2) is magnesium oxide or magnesium hydroxide.
 13. Aconcentrated aqueous liquid composition of matter according to claim 10,wherein reagent (2) is magnesium oxide or magnesium hydroxide.
 14. Aconcentrated aqueous liquid composition of matter according to claim 9,wherein reagent (2) is magnesium oxide or magnesium hydroxide.
 15. Anaqueous liquid composition of matter that has a pH from about 2.5 toabout 8.0 and, when applied as a layer over a surface of a chromiumoxide conversion coating and dried into place thereover, will form asurface on which deionized water has a smaller advancing contact anglethan it has on the surface of the chromium oxide conversion coating,said composition consisting essentially of water and from about 5.0 toabout 40% of a product of reaction between (1) a number of moles,measured as its stoichiometric equivalent as phosphorus atoms, ofphosphoric acid and (2) a total number of moles, measured as itsstoichiometric equivalent as divalent metal atoms, of material selectedfrom the group consisting of (2.1) hydroxides of divalent metals and(2.2) oxides of divalent metals, in amounts of reagents (1) and (2) suchthat the total number of moles of reagent (2) has a ratio to the numberof moles of phosphorus atoms of reagent (1) that is in a range fromabout 1.0:3.5 to about 1.0:1.0.
 16. An aqueous liquid composition ofmatter according to claim 15, said composition having a pH from about3.5 to about 7.0 and consisting essentially of water and from about 6.0to about 30% of a product of reaction between (1) a number of moles,measured as its stoichiometric equivalent as phosphorus atoms, ofphosphoric acid and (2) a total number of moles, measured as itsstoichiometric equivalent as divalent metal atoms, of material selectedfrom the group consisting of (2.1) hydroxides of divalent metals and(2.2) oxides of divalent metals, in amounts of reagents (1) and (2) suchthat the total number of moles of reagent (2) has a ratio to the numberof moles of phosphorus atoms of reagent (1) that is in a range fromabout 1.0:2.6 to about 1.0:1.6.
 17. An aqueous liquid composition ofmatter according to claim 16, said composition having a pH from about4.5 to about 5.5 and consisting essentially of water and from about 8.5to about 11.0% of a product of reaction between (1) a number of moles,measured as its stoichiometric equivalent as phosphorus atoms, ofphosphoric acid and (2) a total number of moles, measured as itsstoichiometric equivalent as divalent metal atoms, of material selectedfrom the group consisting of (2.1) hydroxides of divalent metals and(2.2) oxides of divalent metals, in amounts of reagents (1) and (2) suchthat the total number of moles of reagent (2) has a ratio to the numberof moles of phosphorus atoms of reagent (1) that is in a range fromabout 1.0:2.2 to about 1.0:1.9.
 18. An aqueous liquid composition ofmatter according to claim 17, wherein reagent (2) is magnesium oxide ormagnesium hydroxide.
 19. An aqueous liquid composition of matteraccording to claim 16, wherein reagent (2) is magnesium oxide ormagnesium hydroxide.
 20. An aqueous liquid composition of matteraccording to claim 15, wherein reagent (2) is magnesium oxide ormagnesium hydroxide.